Speaking at the US Capitol at a congressional hearing, Sybiga said: "Right now, we see new attempts by the Kremlin to use nuclear saber-rattling to scare the West.

"Their updated nuclear public rhetoric on the use of nuclear weapons is nothing more than blackmail," he added. "They have used it many times before when strong decisions were made. We must remain cold-headed, clear-eyed, and not give in to fear."

Russian President Vladimir Putin signed a decree on Tuesday broadening the scope of when Moscow can use nuclear weapons, in what was seen as a clear message to the West and Ukraine.

The new doctrine outlines that Russia would consider using nuclear weapons against a non-nuclear state, which would include Ukraine, if they are supported by nuclear powers.

The move came on the same day that Ukraine said it had fired US-supplied long-range missiles into Russian territory for the first time.

Putin had previously warned that authorizing the missiles to be used inside Russia would amount to the US-led NATO alliance being at war with his country.

A spokesman for the US Defense Department, Sabrina Singh, criticized "the same irresponsible (nuclear) rhetoric that we've seen before and that we've seen, frankly, for the past two years."

"It's something that we're going to continue to monitor. But we don't have any indications that Russia is preparing to use a nuclear weapon within Ukraine," she told reporters in Washington on Tuesday.

She said Russia had escalated the conflict "by bringing in another foreign country into the battlefield," referring to what she said were 11,000 North Korean soldiers who had joined Moscow's war effort.

The European Union and Britain both condemned Russia's nuclear policy change and rhetoric from Russian political leaders.